The price doesn’t have to go where you think. A stop-loss is insurance for that case. It closes the trade even if you’re asleep or not looking at your phone.
Example. You bought oil at $70 a barrel and set a stop at $68. The price fell to $68, and the trade closed by itself: −$2 per barrel. Without a stop, the price could have gone to $60, and the loss would have been five times bigger.